Method for fabricating a functionally-graded monolithic sintered working component for magnetic heat exchange and an article for magnetic heat exchange

ABSTRACT

An article for magnetic heat exchange includes a functionally-graded monolithic sintered working component including La 1-a R a (Fe 1-x-y T y M x ) 13 H z C b  with a NaZn 13 -type structure. M is one or more of the elements from the group consisting of Si and Al, T is one or more of the elements from the group consisting of Mn, Co, Ni, Ti, V and Cr and R is one or more of the elements from the group consisting of Ce, Nd, Y and Pr. A content of the one or more elements T and R, if present, a C content, if present, and a content of M varies in a working direction of the working component and provides a functionally-graded Curie temperature. The functionally-graded Curie temperature monotonically decreases or monotonically increases in the working direction of the working component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. divisional patent application claims priority to U.S. utilitypatent application Ser. No. 13/817,304 filed Mar. 12, 2013, which is a371 national phase entry of PCT/IB2011/053629 filed Aug. 17, 2011, whichclaims priority to British Application No. UK 1013784.2 filed Aug. 18,2010, the entire content of which are hereby incorporated by reference.

A method for fabricating a functionally-graded monolithic sinteredworking component for magnetic heat exchange and an article for magneticheat exchange

Practical magnetic heat exchangers, such as that disclosed in U.S. Pat.No. 6,676,772 for example, may include a pumped recirculation system, aheat exchange medium such as a fluid coolant, a chamber packed withparticles of a working material which displays the magnetocaloric effectand a means for applying a magnetic field to the chamber. The workingmaterial can be said to be magnetocalorically active.

The magnetocaloric effect describes the adiabatic conversion of amagnetically induced entropy change to the evolution or absorption ofheat. Therefore, by applying a magnetic field to a magnetocaloricallyactive working material, an entropy change can be induced which resultsin the evolution or absorption of heat. This effect can be harnessed toprovide refrigeration and/or heating.

Magnetic heat exchangers are, in principle, more energy efficient thangas compression/expansion cycle systems. They are also consideredenvironmentally friendly as chemicals such as chlorofluorocarbons (CFC)which are thought to contribute to the depletion of ozone levels are notused.

In practice, a magnetic heat exchanger requires magnetocaloricallyactive material having several different magnetic phase transitiontemperatures in order to provide cooling over a wider temperature range.In addition to a plurality of magnetic phase transition temperatures, apractical working medium should also have a large entropy change inorder to provide efficient refrigeration and/or heating.

A variety of magnetocalorically active phases are known which havemagnetic phase transition temperatures in a range suitable for providingdomestic and commercial air conditioning and refrigeration. One suchmagnetocalorically active material, disclosed for example in U.S. Pat.No. 7,063,754, has a NaZn₁₃-type crystal structure and may berepresented by the general formula La(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z),where M is at least one element of the group consisting of Si and Al,and T may be one or more of transition metal elements such as Co, Ni, Mnand Cr. The magnetic phase transition temperature of this material maybe adjusted by adjusting the composition.

Consequently, magnetic heat exchanger systems are being developed inorder to practically realise the potential advantages provided by thesemagnetocalorically active materials. However, further improvements aredesirable to enable a more extensive application of magnetic heatexchange technology.

An article for magnetic heat exchange is provided that comprises afunctionally-graded monolithic sintered working component comprisingLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) with a NaZn₁₃-typestructure. M is one or more of the elements from the group consisting ofSi and Al, T is one or more of the elements from the group consisting ofMn, Co, Ni, Ti, V and Cr and R is one or more of the elements from thegroup consisting of Ce, Nd, Y and Pr. A content of the one or moreelements T and R, if present, a C content, if present, a H content, ifpresent, and a content of M varies in a working direction of the workingcomponent and provides a functionally-graded Curie temperature. Thefunctionally-graded Curie temperature monotonically decreases ormonotonically increases in the working direction of the workingcomponent.

A working component for an article for magnetic heat exchange isprovided as a monolithic sintered working component with afunctionally-graded Curie temperature. Rather than a single freestandingmonolithic working component that comprises a plurality of layers eachcomprising a differing Curie temperature which increases or decreases ina stepped fashion over the length of the working component, the workingcomponent of the present invention has a functionally-graded Curietemperature gradient that increases or decreases in a smooth, steplessfashion.

The phrases monotonically increasing and monotonically decreasing areused in the mathematical sense and describe a function in which order ispreserved, i.e. an increment is always non-positive or alwaysnon-negative.

Therefore, the working component according to the present inventionfails to include sharp transitions in Curie temperature as is the casefor a working component with a stepped increase or a stepped decrease inCurie temperature. This feature leads to an increase in the efficiencyof the working component and the heat exchange medium is continuouslycooled or heated as it flows in the working direction of the workingcomponent due to the continuously decreasing or increasing Curietemperature.

A sharp transition in Curie temperature is defined as a change of morethan 10° C. over a distance of 0.5 mm.

In a further embodiment, the monotonically increasing or monotonicallydecreasing Curie temperature comprises no non-increasing portions and nonon-decreasing portions, respectively.

In a further embodiment, the Curie temperature gradient, that is Curietemperature per unit length of the working direction, of the workingcomponent is generally linear.

In an embodiment, the functionally-graded Curie temperature decreases orincreases over 80% of a length of the working component with a gradientthat lies within ±50%, or within ±20%, of a linear function of Curietemperature per unit length determined over 100% of the length of theworking component.

The linear function of Curie temperature per unit length is defined asthe difference between the Curie temperature at one end of the workingcomponent and the Curie temperature at the opposing end of the workingcomponent divided by the distance between the two ends. The Curietemperature decreases or increases over a length of the workingcomponent such that this length provides the working direction of theworking component when it is used in a system for magnetic heatexchange.

In order to allow for variations from an exact linear function in apractical sense, the gradient of the Curie temperature over the 80%/o ofthe length of the working component at any one point may lie within±50%, or within ±20% of the perfect linear function of the Curietemperature per unit length determined over 100% of the length of theworking component.

The working component according to the invention, therefore, includes acontinuously increasing or a continuously decreasing Curie temperatureover its length which leads to an increase in efficiency of coolingand/or heating compared to a step-like discontinuous increase ordecrease in Curie temperature.

Due to edge effects, the Curie temperature may only monotonicallyincrease of monotonically decrease over 80% of the total length of theworking component. This 80% or 90% percent of a length the workingcomponent may be centred on the centre of the length of the workingcomponent so that the outermost end portions comprising 10% or 5%,respectively, of the total length have a Curie temperature gradient thatlies outside of this defined function.

Similarly, due to edge effects, the Curie temperature may only increaseor decrease with a gradient that lies within 50% of the linear functionof Curie temperature per unit length over 80% or 90% of the length ofthe working component. This 80% or 90% percent of a length of theworking component may be centred on the centre of the length of theworking component so that the outermost end portions comprising 10% or5%, respectively, of the total length have a Curie temperature gradientthat lies outside of this defined linear function.

A magnetocalorically active material is defined herein as a materialwhich undergoes a change in entropy when it is subjected to a magneticfield. The entropy change may be the result of a change fromferromagnetic to paramagnetic behaviour, for example. Themagnetocalorically active material may exhibit, in only a part of atemperature region, an inflection point at which the sign of the secondderivative of magnetization with respect to an applied magnetic fieldchanges from positive to negative.

A magnetocalorically passive material is defined herein as a materialwhich exhibits no significant change in entropy when it is subjected toa magnetic field.

A magnetic phase transition temperature is defined herein as atransition from one magnetic state to another. Some magnetocaloricallyactive phases such as La_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b)exhibit a transition from paramagnetic to ferromagnetic which isassociated with an entropy change. For these materials, the magnetictransition temperature can also be called the Curie temperature.

The Curie temperature is determined by the composition of themagnetocalorically activeLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) phase which has aNaZn₁₃-type structure. In particular, the Curie temperature may bedetermined by selecting the elements T and/or R and/or M and/or C.

In a further embodiment, the Curie temperature may also be selected byincluding hydrogen into the magnetocalorically activeLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) phase. The hydrogencontent may also vary along the length of the working component in orderto provide a varying Curie temperature along the length of the workingcomponent. In further embodiments, the hydrogen content is sufficientlyuniform along the length of the working component that the variation inthe further elements R, T and M results in the varying Curietemperature.

The content of the element M may be adjusted and, therefore, may varydepending on the type of the element T and R and/or the amount of theelement T and R. The content of the element M may be adjusted in orderto provide a sinter activity that is similar throughout the length ofthe working component despite the varying composition and varying Curietemperature. This has the effect of producing a density that is similarthroughout the length of the working component despite the varyingcomposition and varying Curie temperature.

By having a similar sinter activity throughout the length of the workingcomponent, cracks and delamination of portions of the working componentcan be avoided and differing Curie temperatures can be provided within asingle monolithic sintered working component.

In one particular embodiment, M is silicon and the silicon content isadjusted and, therefore, may vary depending on the type of the element Tand R and/or the amount of the element T and R in order to provide asinter activity that is similar throughout the length of the workingcomponent.

The functionally-graded Curie temperature may have an undulatingstructure about a perfectly linear function. Therefore, in a furtherembodiment, the functionally-graded Curie temperature may increase ordecrease over 80% of the length of the working component with a gradientthat lies within ±20% or ±10% of the linear function of Curietemperature per unit length determined over 100% of the length of theworking component.

The average Curie temperature gradient may lie within 5° C. permillimetre to 0.5° C. per millimetre over 80% of the length workingcomponent. For a working component of 20 mm, this average Curietemperature gradient provides an effective cooling range of between 100°C. to 10° C.

As discussed above, the M content, e.g. the silicon content, of theworking component varies along the length of the working component inorder to provide a sinter activity which is more uniform along thelength of the working component despite the different contents of theelements T and R and C, if present, along the length of the workingcomponent providing the functionally-graded Curie temperature. Theuniform sinter activity provides a working component with a uniformdensity along the length of the working component.

In an embodiment, the working component has a density d in a definedportion of 5 volume percent to 10 volume percent of the total volume ofthe working component. This density d of the defined portion lies withina range of ±5% or ±2% of an average total density, d_(av), of theworking component.

The M content, x, may lie within the range of 0.05 to 0.2 throughout thevolume of the working component. If M is silicon, the silicon content,x, may lie within the range of 0.05 to 0.2 throughout the volume of theworking component.

In a particular embodiment, the monolithic working component comprisesCo and/or Mn and the silicon content, Si_(act), lies within ±5% ofSi_(m), wherein Si_(m)=3.85−0.0573×Co_(m)−0.045×Mn_(m) ²+0.2965×Mn_(m),wherein Si_(m) is the metallic weight fraction of silicon, Mn_(m) is themetallic weight fraction of manganese, Co is the metallic weightfraction of cobalt.

As used herein, the subscript m denotes the metallic weight fraction.The metallic weight fractions are calculated depending on the oxygen andnitrogen content. The metallic weight fraction is defined herein as theresult of a calculation separating and removing the rare earth, RE,content which is bonded in the form of RE oxides and RE nitrides fromthe total composition according to the following formulas (for RE=La):

La₂O₃ = 6.79^(*)O LaN = 10.9^(*)N$f = \frac{100}{100 - {{La}_{2}O_{3}} - {LaN}}$

Consequently,La_(m)=(La−5.8*O−9.9*N)*fSi_(m)=Si*fAl_(m)=Al*fCo_(m)=Co*fMn_(m)=Mn*fwhere the subscript m denotes the metallic weight fraction and La, O, N,Si, Al, Co and Mn and so on denote the weight percent of this element.

In a first approximation, the metallic RE content can also be calculatedfor La-rich alloys as:

${RE}_{m} = {\left( {{RE} - {5.8^{*}O} - {9.9^{*}N}} \right) \times \frac{100}{100 - {6.8*O} - {10.9*N}}}$

For Si, Co, Mn and so on, the metallic contents are close to the totalcontent as the factor f is around 1.02. However, for the RE element,there is a larger difference. For example, in the embodiments describedhere, a content of around 18 wt % La is used to provide a metalliccontent of 16.7 wt % which corresponds to the stoichiometry of the 1:13phase.

Cobalt and/or manganese contents and the silicon content which aredefined as above provide differing Curie temperatures by varying theamount of cobalt and/or manganese. A uniform sinter activity and,therefore, density in the final working component is achieved by varyingthe silicon content according to the weight fractions of cobalt andmanganese, if present.

In a further embodiment, Si_(act) lies within +2% of Si_(m).

In a particular embodiment, the element R is optional and the element Tis present such that 0≤a≤0.5 and 0.003≤y 0.2.

In a further particular embodiment, the element R is present and theelement T is optional such that 0.05≤a≤0.5 and 0≤y≤0.2.

In a further particular embodiment, both T and R are present such that0.05≤a≤0.5 and 0.003≤y≤0.2.

The element carbon may also be included. It is thought that carbon isaccommodated interstitially in the NaZn₁₃-type structure. However, it isalso possible that some, if not all, of the carbon may be accommodatedon lattice sites of the NaZn₁₃ structure. The carbon content may be0≤b≤1.5 or 0<b≤1.5 or 0.05≤b≤0.5.

The element hydrogen may also be included. It is thought that hydrogenis accommodated interstitially in the NaZn₁₃-type structure. Thehydrogen content may be 0≤z≤3 or 1.4<z≤3.

In one embodiment, the hydrogen content is kept as high as possible, forexample as close as possible to the saturation limit of hydrogen in themagnetocaloric active phase.

In one particular embodiment, the magnetocaloric activeLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) phase includes only theelement T and, in particular where the element T is cobalt or nickel,the magnetocalorically active phase is free of hydrogen.

In a further particular embodiment, T is one or more of the 30 groupconsisting of Mn, Ti, V and Cr and R is one or more of the groupconsisting of Ce, Nd and Pr and a hydrogen content of 1.4≤z≤3 isincluded.

This embodiment may be used if the Curie temperature range and,therefore, the working temperature range of the working componentincluding one or more of the elements Mn, Ti, V and Cr and R is one ormore of the group consisting of Ce, Nd and Pr is too low. By includinghydrogen, the Curie temperature of all of the different compositions ofthe working component is increased so that the working temperature rangeof the working component is shifted to a higher temperature, withoutlosing the advantages of the generally linear increase or decrease inthe Curie temperature.

In a further embodiment, the hydrogen content of the monolithic workingcomponent monotonically increases or monotonically decreases over 80% ormore of the working component and provides a functionally-graded Curietemperature that increases or decreases monotonically over 80% or moreof the working component.

For some applications, a hydrogen content of less than a thresholdvalues, for example 90% of the hydrogen saturation value, can lead tothe NaZn₁₃-phase having a stability that is insufficient for theparticular application. In these embodiments, the hydrogen content isvaried within the range considered to give suitable stability of theNaZn₁₃-phase.

A method for fabricating a functionally-graded monolithic sinteredworking component for magnetic heat exchange is provided that comprisesthe following. Powder comprising elements in amounts suitable to form aLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) phase with a NaZn₁₃-typestructure is provided, wherein T is one or more of the elements from thegroup consisting of Mn, Co, Ni, Ti, V and Cr, M is Si and, optionally,Al, and R is one or more of the elements from the group consisting ofCe, Nd, Y and Pr. The powder is formed to provide a green body such thatthe amount of the one or more elements T and R, the amount of C, ifpresent, and the amount of M varies in a pre-determined direction of thegreen body. The green body is sintered and subsequently heat treated ata temperature T_(diff) and for a time t selected to allow diffusion ofone or more of the elements T or R and C and a working component isformed that comprises a functionally-graded Curie temperature thatmonotonically increases or monotonically decreases in the pre-determineddirection.

The monolithic sintered working component has a functionally-gradedCurie temperature that is formed by diffusion. Since thefunctionally-graded Curie temperature is formed by diffusion, the formor shape of the Curie temperature variation along the length of theworking component is more uniform in a smooth continuous sense than inthe green body which may comprise individual layers of very differentcompositions with very different Curie temperature. For example,adjacent layers of the green body may have a difference in Curietemperature of 1° C. to 50° C.

After the diffusion reaction, the individual layers no longer have sharpinterfaces between them and may even no longer be distinguishable sothat the Curie temperature increases or decreases in a continuous mannerfrom one end of the working component to the other. Such a monolithicfunctionally-graded working component has the advantage that the heatexchange efficiency of the working component is improved as the Curietemperature monotonically increases or monotonically decreases along thelength of the working component as the working medium is continuouslyheated or cooled, respectively.

In an embodiment, the temperature T_(diff) and the time t are selectedto provide a functionally-graded Curie temperature that decreases orincreases with a gradient that lies within ±50% of a linear functionover 80% of a length of the working component. In this embodiment, thefunctionally-graded Curie temperature has a generally linear form thatlies at most from perfectly linear function of ±20%, or ±10%. Theefficiency of the heat exchange may be further increased the more linearthe decrease or increase of the Curie temperature.

The monolithic sintered working component may be fabricated by heatingthe working component at a temperature T_(diff) and for a time t toallow the diffusion of one or more of the elements T, R and C, ifpresent. The functionally-graded Curie temperature is produced bydiffusion of the elements so as to provide the working component with acontinuously varying composition along its length and a monotonicallyvarying Curie temperature along its length.

Such a diffusion reaction can, in principle, be produced by providing atleast two portions, each comprising a magnetocaloric active phase orelements having a suitable stoichiometric ratio to form a magnetocaloricactive phase and differing contents of T, R and/or carbon to provide adiffering Curie temperature. The two or more portions may be placed incontact with one another and heat treated for sufficient time thatdiffusion between the two portions occurs thus joining the portionstogether and producing a gradient of the elements T, R and/or carbon anda functionally-graded Curie temperature within a single workingcomponent.

The diffusion length required to produce a monotonically increasing ordecreasing Curie temperature depends on the size of the portions. Asused herein, macroscopic diffusion is used to describe a portion length,1, of at least 0.5 mm and a diffusion length of 1, i.e. at least 0.5 mm.Microscopic diffusion is used to describe a portion length, 1, of lessthan 0.5 mm and a diffusion length of less than 0.5 mm.

The temperature, T_(diff), and the time, t, may be chosen to produce adiffusion length of around the length of the portions. For example, ifthe two portions of differing Curie temperature have a length of 10 mm,T_(diff) and t may be selected so that the elements, T, R and C, have adiffusion length of 10 mm.

If the two or more portions provide a stacked layered structure in thegreen body, the layers may have a thickness of 10 μm to 50 μm, forexample. In this example, T_(diff) and the time t may be selected sothat the elements, T, R and C, have a shorter diffusion length of 10 μmto 50 μm.

In an embodiment, the mean composition of a portion having a length, 1,may be defined by the ratio of numbers of alternating layers withdifferent compositions. For example, a first portion could include alayered structure with two layers of composition A alternating with onelayer of composition B to give a ratio A:B of 2:1 and a second portioncould include a layered structure with one layer of composition Aalternating with one layer of composition B to give a ratio A:B of 1:1.In these embodiments, the diffusion length is comparable to the totallength of each portions, 1, and not the smaller thickness of theindividual layers making up the portion.

In the case of powders of differing composition being mixed in varyingproportions in order to provide the varying composition in the workingdirection of the working component, the diffusion length may be evenshorter if the particle size of the powders is less than 10 μm.

The temperature T_(diff) and/or the time t used for the diffusionreaction may be selected so as to provide a particular diffusion rate ofthe elements T and/or R and/or carbon. In a particular embodiment, thetemperature T_(diff) is selected to provide a diffusion rate of theelements T and/or C of at least 2×10⁻¹¹ m²/s. In a further particularembodiment, the temperature T_(diff) is selected to provide a diffusionrate of the element C of at least 1×10⁻¹⁰ m²/s.

The temperature T_(diff) may be 900° C.≤T_(diff) 1200° C. or 1050°C.≤T_(diff)≤1150° C. and/or the time t may be 1 h≤t≤100 h. Generallyspeaking, the time may be reduced for higher temperatures in order toachieve a preselected degree of diffusion.

The varying composition of the green body may be provided in differentways.

In an embodiment, a plurality of powders is provided which comprisediffering R, T, M and C contents selected to provide differing Curietemperatures when suitably heat treated to form the NaZn₁₃ structure.

Two or more basic powders having differing compositions may be weighedand mixed and then differing proportions of these two or more basicpowders may be mixed together to provide yet further intermediatecompositions. This approach may be useful if the basic powders areproduced by milling a solidified melt in order to reduce the number ofdifferent compositions which have to be produced by melt casting.

In order to form the green body from the plurality of different powders,layers of the plurality of powders may be stacked such the content of R,T, M and/or C increases or decreases in a direction of the stack. Inother words, a stack of layers of differing composition is built upvertically such that the content of the elements R, T, M and/or Cincreases or decreases in a vertical direction.

The plurality of powders may also be mixed with a liquid and,optionally, a binder and/or a dispersant to form a plurality of slurriesor pastes of differing composition. According to one example embodiment,the viscosity of the slurry or paste is between 200 mPas and 100,000mPas. These slurries or pastes may then be sequentially applied to forma stack in which the content of the elements R, T, M and/or C increasesor decreases in the direction in which the stack is built up. Theslurries or pastes may be applied by screen printing or doctor-blading.

The use of slurries or pastes may be useful to apply thinner layers, forexample layers having a thickness of 10 μm to 60 μm. The use of thinnerlayers of differing composition may be useful in reducing the diffusiontime between adjacent layers which leads to produce a suitably linearCurie temperature gradient.

In an embodiment, the composition is varied in the stacking direction byvarying the numbers of layers of differing composition applied to thestack in the stacking direction. In this way the average composition maybe varied in the stacking direction using a more limited number ofslurries of differing composition. For example, the followingarrangement could be used: 5 layers of slurry A, 3 layers of slurry B, 3layers of slurry A, 3 layers of slurry B, 1 layer of slurry A and 5layers of slurry B.

If a liquid and/or binder and/or plasticizer are used, it may be removedbefore the green body is sintered by heat treatment, for example at atemperature of less than 500° C.

In a further embodiment, varying proportions of the powders are mixedwith one another before being arranged in a former such that the contentof R, T, M or C of the powder in the former increases or decreases overa length of the former. Powders may be mixed with one another byintroducing varying proportions of powders of differing composition intoa sieve/conveyor structure which shakes the powders, mixing them withone another before depositing them in the former. The composition mayvary parallel to a height of the former or along the length of anelongate former.

In a further embodiment, varying proportions of the powders areintroduced in the former such that the content of R, T, M or C increasesor decreases in the insertion direction. In this embodiment, the powdersof differing composition and in varying proportion are introduceddirectly in the former rather than being mixed with one another beforebeing placed in the former. The ratio of the powders may be variedduring the filling process gradually resulting in a monotonic variationof the mean composition, thus reducing the required diffusion times.

The powder may be formed to a green body by applying pressure, forexample by die pressing or by isostatic pressing. After the pressure isapplied, the green body may be sintered at a temperature of 900° C. orabove to densify the green body. In an embodiment, the temperature ischosen so that a density of greater than 90%, of the theoretical densityis achieved in the working component.

The amount of M may be adjusted depending on the type of the element Tand/or R as well as adjusted depending on the amount of the element Tand/or R in order to provide a sinter activity for the particularcomposition which is similar to the sinter activity of the othercompositions used to produce the working component. This provides amonolithic working component with a similar density throughout itsvolume despite the differing composition.

In an embodiment, Co and/or Mn are present, M is Si and the amount of Siis selected according to Si_(m)=3.85−0.0573×Co_(m)−0.045×Mn_(m)²+0.2965×Mn_(m), wherein Si_(m) is the metallic weight fraction ofsilicon, Mn_(m) is the metallic weight fraction of manganese and Co_(m)is the metallic weight fraction of cobalt.

In a further embodiment, Mn and/or Ce(MM) are present, M is Si and theamount of Si is selected according to Si_(m)=3.85−0.045×Mn_(m)²+0.2965×Mn_(m)+(0.198−0.066×Mn_(m))×Ce (MM)_(m), wherein Si_(m) is themetallic weight fraction of silicon, Mn_(m) is the metallic weightfraction of manganese and Ce(MM)_(m) is the metallic weight fraction ofcerium misch metal.

In a further group of embodiments, the working component is furtherhydrogenated after sintering.

The La_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z) phase has a NaZn₁₃-typestructure and, if it includes hydrogen, the hydrogen atoms are thoughtto occupy interstitial sites in the NaZn₁₃-type structure. The hydrogencan be introduced into these interstitial sites after formation of theLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃ phase. The Curie temperature of asubstantially fully hydrogenated ternary La(Fe, Si)₁₃H_(z) phase may bearound +85° C. The Curie temperature of theLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z) phase may be adjusted byadjusting the hydrogen content as well as by substitution of metallicelements for La and Fe.

Hydrogenation may be performed by heat treating the working componentunder a hydrogen partial pressure of 0.5 to 2 bars. The hydrogen partialpressure may be increased during the hydrogenation heat treatment. Thehydrogenation may comprise heat treating at a temperature in the rangeof 0° C. to 100° C. and, preferably, in the range 15° C. to 35° C. Afinal heat treatment at temperatures of less than 100° C. in a hydrogenatmosphere, preferably at 0.5 to 2 bars has been found to reliablyproduce working components with the hydrogen content, z, of at least 90%of the hydrogen saturation value, z_(sat).

In further embodiments, the hydrogenation comprises a dwell at atemperature T_(hyd), wherein 400° C.≤T_(hyd)≤600° C. and may comprise adwell at a temperature T_(hyd) in the range of 400° C.≤T_(hyd)≤600° C.followed by cooling in a hydrogen atmosphere to a temperature of lessthan 100° C.

In further embodiments, the working component is only subjected tohydrogen gas above a threshold temperature. In one embodiment, thehydrogenation comprises heating the working component from a temperatureof less than 50° C. to at least 300° C. in an inert atmosphere andintroducing hydrogen gas only when a temperature of at least 300° C. isreached. The working component is maintained in a hydrogen containingatmosphere at a temperature in the range 300° C. to 700° C. for aselected duration of time, and cooled to a temperature of less than 50°C. to provide a second working component. This method has been found toresult in second working components with a hydrogen content, z, of 90%or more of the hydrogen saturation content, z_(sat), and also inmechanically stable second working components.

In further embodiments of a method in which the working component issubjected to hydrogen only at temperatures above a thresholdtemperature, the working component may be cooled to a temperature ofless than 50° C. in a hydrogen-containing atmosphere.

In particular, it is found that if hydrogen is first introduced attemperatures lower than around 300° C., the working component maydisintegrate into pieces or at least lose its previous mechanicalstrength. However, these problems may be avoided by first introducinghydrogen when the working component is at a temperature of at least 300°C.

Alternatively, or in addition, hydrogen gas is introduced only when atemperature of 400° C. to 600° C. is reached. After hydrogenation, theworking component may comprise at least 0.18 wt % hydrogen.

In further embodiments, the hydrogen content of the working componentmonotonically increases or monotonically decreases along 80% or more ofthe working component so as to produce a Curie temperature gradient thatmonotonically increases or monotonically decreases.

The hydrogen content may be varied as a function of position along theworking direction of the working component by heat treating ahydrogenated working component in a temperature gradient so as to removehydrogen from the working component as a function of the temperature andof position if the working direction extends in the direction of thetemperature gradient.

The end of the working component placed at the higher temperature end ofthe temperature gradient has a lower hydrogen content after the heattreatment due to a higher hydrogen loss compared to the opposing end ofthe working component that was placed at the lower end of thetemperature gradient and was subjected to a lower hydrogen loss.

A multi-step heat treating process may also be used to heat treat thepowder mixture and produce the working component. In an embodiment, themulti-step heat treatment comprises a first dwell at T_(sinter) for atime t₁ in vacuum and a time t₂ in argon, followed by cooling to atemperature T₁, wherein T₁<T_(sinter), followed by a second dwell at T₁for a time t₃ followed by rapid cooling. Typical parameter ranges forsuch a multi-step heat treatment may be 900° C.≤T_(sinter)≤1200° C.,900° C.≤T₁≤1080° C. and/or 0.5 h≤t₁≤10 h and/or 0.5 h≤t₂≤10 h and/or 1h≤t₃≤20 h and/or rapid cooling at a rate of 5° C./min to 200° C./min.

Embodiments and specific examples will now be described in connectionwith the following figures and Tables.

FIG. 1 illustrates a green body according to a first embodiment.

FIG. 2 illustrates a working component for magnetic heat exchangefabricated from the green body of FIG. 1.

FIG. 3 illustrates a typical curve of temperature change as a functionof temperature for a single slice of a working component.

FIGS. 4A-4C illustrate graphs of Curie temperature, maximum temperaturechange and full width at half maximum value as a function of theposition of the slice in a first working component.

FIGS. 5A-5C illustrate graphs of Curie temperature, maximum temperaturechange and full width at half maximum value as a function of theposition of the slice in a second working component.

FIGS. 6A-6C illustrate graphs of Curie temperature, maximum temperaturechange and full width at half maximum value as a function of theposition of the slice in a third working component.

FIGS. 7A-7C illustrate graphs of Curie temperature, maximum temperaturechange and full width at half maximum value as a function of theposition of the slice in a fourth working component.

FIG. 8 illustrates a definition of two different Curie temperaturegradients and of the diffusion zone used to establish the values givenin Table 4.

FIG. 9 illustrates a schematic diagram of the concentration profile overthe working component at t=0.

FIG. 10 illustrates the measured Curie temperature as a function ofposition in a first working component.

FIG. 11 illustrates a graph of measured Curie temperature as a functionof position for a second working component.

FIG. 12 illustrates a graph of Curie temperature as a function ofposition for a third working component.

FIG. 13 illustrates a graph of entropy change as a function oftemperature for differing carbon contents.

FIGS. 14A-14C illustrate graphs of Curie temperature, entropy change asa function of carbon content and entropy change as a function of Curietemperature.

FIGS. 15A and 15B illustrate measured Curie temperatures as a functionof silicon content and carbon content.

FIGS. 16A and 16B illustrate maximum entropy change as a function ofsilicon content and carbon content.

FIG. 17 illustrates a graph of sinter density in dependence of sintertemperature for samples including varying Si and Al contents.

FIG. 18 illustrates a graph of sinter density in dependence of sintertemperature for samples with varying aluminium contents.

FIG. 19 includes Table 1 which illustrates the composition of threestarting powders with a cobalt composition selected to give differingCurie temperatures.

FIG. 20 includes Table 2 which illustrates the Curie temperatures offour multilayer green bodies.

FIG. 21 includes Table 3 which illustrates sinter and diffusion heattreatments.

FIG. 22 includes Table 4 which summarizes the measured diffusion zonesand Curie temperature gradient.

FIG. 23 includes Table 5 which summarizes the calculated diffusioncoefficient of cobalt in the working components.

FIG. 24 includes Table 6 which summarizes the compositions of the threestarting powders of the second set of examples.

FIG. 25 includes Table 7 which summarizes the composition of multi-layergreen bodies were fabricated from layers of the powders of Table 6.

FIG. 26 includes Table 8 which summarizes the density of green bodiesheat treated at 1120° C. for 4 hours, 16 hours or 64 hours.

FIG. 27 includes Table 9 which summarizes the compositions of examplesof differing carbon content.

FIG. 28 includes Table 10 which summarizes the sinter temperatures andthe density of the samples of Table 9 after heat treatment at differenttemperatures.

FIG. 29 includes Table 11 which summarizes the compositions of a set ofexamples in which carbon is substituted for silicon.

FIG. 30 includes Table 12 which summarizes the compositions of sixsamples of differing carbon and silicon content.

FIG. 31 includes Table 13 which summarizes the sinter temperature anddensity measured for the samples of Table 12.

FIG. 32 includes Table 14 which summarizes the compositions of sixsamples of varying Si and Al content.

FIG. 33 includes Table 15 which summarizes the density of the samples ofTable 14 after heat treatment at different temperatures.

FIG. 34 includes Table 16 which summarizes the compositions of sixsamples of varying Al content.

FIG. 35 includes Table 17 which summarizes the density of the samples oftable 16 after heat treatment at different temperatures.

FIG. 1 illustrates a green body 1 according to a first embodiment whichmay be heat treated to form a working component for a magnetic heatexchanger which includes a functionally-graded Curie temperature.

The green body 1 includes three portions 2, 3, 4. Each portion includeselements having a stoichiometry suitable to form aLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃ phase. The composition of eachportion differs from that of the others and is selected to give apreselected Curie temperature, T_(c). In one particular example, theCurie temperature of the first portion 2 is 60° C., the Curietemperature of the central portion 3 is 30° C. and the Curie temperatureof the third portion 4 is 15° C. The green body 1 therefore has threeseparate portions with a composition selected to produce a Curietemperature which decreases along the length of the green body 1 in astep like fashion as is illustrated in FIG. 1.

The Curie temperature of the three portions can be preselected byselecting the composition of the cobalt content of theLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)Si_(x))₁₃ phase according to the followingequation:T _(c)=14.82×Co_(m)−87.1  (1)wherein Co_(m) is the metallic weight fraction of cobalt.

Additionally, the silicon content of the three portions 2, 3, 4 of thegreen body 1 is adjusted so that each of the portions has a sinteractivity that is similar. The silicon content is selected according tothe following equation:Si_(m)=3.85−0.0573×Co_(m)  (2)wherein Si_(m) is the metallic weight fraction of silicon and Co_(m) isthe metallic weight fraction of cobalt.

The similar sinter activity enables the three portions 2, 3, 4 of thegreen body 1 to be formed as integral parts of a single monolithicsintered working component after heat treatment of the green body 1. Amonolithic working component having a plurality of Curie temperatureswhich decrease over the length of the working component is provided.

In order to form a monolithic working component from the green body 1,the green body 1 is sintered. The sintered body 1 is subsequently heattreated at a temperature T_(diff) which lies within the range of 900° C.to 1200° C. for a time within a range of 1 hour to 100 hours. Thetemperature and the time of the heat treatment are selected so as toallow diffusion of one or more elements along the length of the greenbody 1 and form a working component 7.

In particular, a diffusion zone 8, 9 is created at the interfaces 5, 6between adjacent portions 2, 3 and 3, 4 respectively as a result of theheat treatment, as is illustrated in FIG. 2. The Curie temperature ofthe working component 7 when viewed as a function of the length nolonger has a stepped structure as in the green body illustrated in FIG.1 but has a smooth undulating decrease which is generally linear, as isillustrated in FIG. 2. The Curie temperature monotonically decreasesfrom left to right in the view of FIG. 2.

The heat treatment conditions may be selected so that this decrease inthe Curie temperature is as linear as possible. The heat treatmenttemperature and the time may be selected depending on the diffusioncoefficients of the particular elements included in the basic LaFe₁₃phase which is or are used to adjust the Curie temperature and dependingon the thickness of the portions of differing composition included inthe green body.

As a Curie temperature gradient is desirable across the length of theworking component, the diffusion time should not be selected to be toolong as eventually the entire working component would have the samecomposition and a uniform Curie temperature across its length.

By providing a monolithic sintered working component 7 with afunctionally-graded Curie temperature which decreases in a generallylinear fashion from one end 10 of the working component 7 to the otherend 11 of the working component 7, the efficiency of the heat exchangecan be increased over that possible by a stepped decrease in the Curietemperature as, for example, illustrated in FIG. 1.

In a first set of embodiments, the Curie temperature of the workingcomponent was adjusted by adjusting the cobalt content. Table 1illustrates the composition of three starting powders with a cobaltcomposition selected to give a Curie temperature of 10° C., 35° C. and60° C. The silicon composition was also selected according to equation(2) above.

Four green bodies were formed including two or three of these powders ina layered structure. The Curie temperatures of the three green bodiesare summarised in Table 2. The first green body is fabricated usingpowders having a composition selected to provide a Curie temperature of10° C. and 60° C. The second green body is fabricated from the powdershaving a composition selected to have a Curie temperature of 10° C. and35° C. The third green body is fabricated using three powders having thediffering Curie temperatures of 10° C., 35° C. and 60° C. The fourthgreen body is fabricated using powders having compositions for the twoCurie temperatures of 35° C. and 60° C.

In each case, the different powders were placed in layers in a press.The powder with the lowest Curie temperature was placed in the pressfirst and pressed with a pressure of around 0.15 tonne/cm². A flatsurface was produced on which the second powder was introduced withoutmixing of the two powders. After the layers were built, the green bodywas then pressed with a pressure of 2 tonne/cm².

The green bodies were given a sintering and diffusion heat treatment at1100° C. for 4 hours, 16 hours or 64 hours as summarized in Table 3.These times were chosen so that the diffusion length is expected to bedouble that of the previous time as the diffusion distance/length isgenerally expected to be proportional to the square root of thediffusion time. In particular, the green bodies were heated to 1100° C.for 3 hours in a vacuum and the remaining dwell time in argon beforefurnace cooling to 800° C. where the temperature was held for a further8 hours before fast cooling to room temperature.

The annealing at 800° C. was performed in order to decompose the La(Fe,Co, Si)₁₃ phase in order to allow machining of the working componentwithout forming undesirable cracks. This method uses the teaching of thepublished application WO2010/038099 A1 which is hereby incorporated byreference in its entirety.

In order to investigate the diffusion between the different portions ofthe working components, the working components were cut into slices witha spark erosion technique. The working components were cut so that someslices extend over the length of the working component. Others were cutin a perpendicular direction so that slices along the length of theworking component could be tested to establish the Curie temperature ofthis particular slice. The individual slices were around 1 mm thick andthe total length of the working component was around 25 mm.

To recombine the magnetocalorically activeLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)Si_(x))₁₃ phase, the slices were heattreated at 1050° C. for 4 hours and quickly cooled.

The Curie temperature of the slices was measured with an infraredmonitor whilst the test sample rotated in a magnet system so that it wassubjected to an alternating external field of 1.6 T and 0 T. FIG. 3illustrates a typical curve of temperature change as a function oftemperature for a single slice.

The peak temperature, which may also be described as the Curietemperature, the maximum temperature change and the full width at halfmaximum value of the curves is illustrated as a function of the positionof the slice in the working component for the working components 1, 2, 3and 4 in FIGS. 4, 5, 6 and 7, respectively.

Diffusion between the layers of differing composition is seen for eachof the working components as the form of the decrease in the Curietemperature becomes smoother and more linear for increasing heattreatment time. The plateaus become shorter and less sharply defined forincreasing diffusion times.

Table 4 includes a summary of the measured diffusion zones and Curietemperature gradient of these samples.

Two different ways of establishing the Curie temperature gradient areused. FIG. 8 illustrates the two different Curie temperature gradientsand the definition of the diffusion zone use to establish the valuesgiven in Table 4.

The gradient illustrated with the solid line in FIG. 8 is established bymeasuring the gradient of the central portion of the diffusion zone byestablishing the best fit of this portion of the curve.

The mean gradient, illustrated in FIG. 8 by the dashed line, is theslope of the line connecting the two points in the end zones at whichthe peak temperature starts to deviate from a constant value. These twoendpoints were also used to establish the diffusion zone.

The results may be used to calculate the diffusion coefficient of cobaltin these working components. The calculated values are summarised inTable 5.

The following equation was used which is applicable only for diffusioncoefficients which are independent of the concentration of the diffusingelement. This factor was assumed in this case. It was assumed that theconcentration on the left is c₁ and that on the right is c₂ and theconcentration profile over the working component is known at start pointof t=0 as illustrated in FIG. 9 and summarised in the followingequation:

$\begin{matrix}{\frac{\partial c}{\partial t} = {D\frac{\partial c^{2}}{\partial x^{2}}}} & (2)\end{matrix}$

The above differential equation can be solved as follows:

$\begin{matrix}{{c\left( {x,t} \right)} = {\frac{c_{1} + c_{2}}{2} + {{\frac{c_{1} - c_{2}}{2} \cdot {erf}}\frac{(x)}{\sqrt[2]{Dt}}}}} & (3)\end{matrix}$where erf is an error function, t the diffusion time and D is theinterdiffusion coefficient.

Table 5 illustrates that the calculated interdiffusion coefficients varyonly slightly over the nine samples. The average value is 2.3×10⁻¹¹m²/s.

In a further set of embodiment, a varying carbon content was used toadjust the Curie temperature.

The compositions of the three starting powders are illustrated in Table6. As in the first set of embodiments, the composition of the threepowders was selected so as to produce a Curie temperature of 10° C., 35°C. or 60° C. The carbon content increases from 0.06 weight percent, 0.36weight percent and 0.68 weight percent for the three powders,respectively.

Green bodies were fabricated from layers of two or three of thesepowders. The compositions are summarised in Table 7. The fifth greenbody has a layer with a Curie temperature of 10° C. and a second layerwith the Curie temperature of 35° C.

The sixth green body has a first layer having a Curie temperature of 10°C. and a second layer having a Curie temperature of 60° C.

The seventh green body has three layers having the Curie temperatures10° C., 35° C. and 60° C., respectively.

As in the first set of embodiments, the powder with the lowest Curietemperature was placed in the press first, pressed to form a flatsurface before the second powder was placed into the press. In case ofthe seventh green body with three different differing Curietemperatures, the second layer was pressed to form a flat surface beforethe powder forming a third layer was placed into the press.

These green bodies were given a sintering and diffusion heat treatmentat 1120° C. for 4 hours, 16 hours or 64 hours. The density of theworking components after heat treatment of the green bodies issummarised in Table 8.

In this embodiment, the sinter activity was not adjusted for thediffering carbon contents by adjusting the silicon content. The workingcomponent fabricated from the second sort of powder and heated for 64hours split into two pieces. It is though that this is a result of thesinter activity being sufficiently different for the two portions ofvarying carbon content. These results in combination with the sinterdensity achieved for other carbon and silicon contents, which aresummarized in Table 10 and for examples 1 to 3 of Table 13, indicatesthat the silicon content should be reduced for increased carboncontents.

If aluminium is used instead of silicon, it is expected that thealuminium content also has to be reduced for increasing carbon contentas can be seen from the results summarized in Table 16 and Table 17.

FIG. 10 illustrates the measured Curie temperature as a function ofposition for the fifth working component heat treated at 4 hours, 16hours and 64 hours. FIG. 10 illustrates that the gradient of the Curietemperature (Curie temperature per unit length) becomes more linear witha less pronounced curve across the interface between the two portions ofthe green body with increasing diffusion time.

FIG. 11 illustrates a graph of measured Curie temperature as a functionof position for the working component fabricated from the sixth greenbody which was heated at 1120° C. for 16 hours.

FIG. 12 illustrates a graph of Curie temperature as a function ofposition for a working component made from the seventh green body whichwas heated for 16 hours and 64 hours.

FIGS. 10 to 12 illustrate that diffusion between the layers havingdifferent carbon compositions has occurred to give a functionally-gradedCurie temperature.

The following set of examples was carried out in an attempt to establishwhether carbon is accommodated in the NaZn₁₃ structure in interstitialpositions or at lattice positions.

In the first set of examples, a composition including 16.7 weightpercent lanthanum, 3.48 weight percent silicon, 6.55 weight percentcobalt and 73.250% iron was fabricated. This composition should have aCurie temperature of 10° C. In this composition all of the lattice sitesshould be occupied by these elements.

0.4 weight percent carbon was introduced into a portion of this powderin the form of graphite powder and mixed for 30 minutes with steelballs. This graphite containing powder was mixed in varying proportionswith non-graphite containing powder to produce examples of differingcarbon content. The compositions are summarised in Table 9.

The samples were heated at one or more of five heat treatment conditionsat varying temperatures for 3 hours in vacuum and 1 hour in argon beforefast cooling. The sinter temperatures and the density of the samplesheated at different temperatures are summarised in Table 10. In order toachieve the highest density, the sinter temperature may be increased forincreasing carbon contents.

The entropy change as a function of temperature for differing carboncontents is summarised in FIG. 13. As the carbon content increases, thepeak temperature, which corresponds to the Curie temperature, increases.The relationship between the Curie temperature, entropy change andcarbon content and the relationship between entropy change and Curietemperature are summarised in FIG. 14.

In a further set of examples, carbon was substituted for silicon in aLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)Si_(x))₁₃-based composition. Thecompositions are summarised in Table 11 and varying portions of thepowders were mixed to form six samples of differing carbon and siliconcontent. The compositions of the six samples are summarised in Table 12.In this set of embodiments, it was attempted to establish whether carboncould replace silicon in the NaZn₁₃ structure.

Samples were heated at various sintering temperatures for 3 hours invacuum followed by 1 hour in argon before fast cooling. The sintertemperature and density measured are summarised in Table 13.

FIG. 15 illustrates the measured Curie temperatures as a function ofsilicon content and carbon content.

FIG. 16 illustrates the maximum entropy change in an external field of1.6 T for the samples as a function of silicon content and carboncontent.

Samples 1 to 3 illustrate a dependence of the Curie temperature on thesilicon and carbon content which can be summarised 30 by the followingequation:T _(c)=97×C−87.1  (4)wherein C is the carbon content in weight percent.

This is similar to that found in the first set of embodiments. However,samples 4 to 6 are found to exhibit different behaviour and a decreasein Curie temperature. This indicates that carbon does not take the samesites as Si, but is incorporated only on interstitial sites, as is thecase for hydrogen. The results also indicate that at least 3.5 weightpercent silicon is required to stabilize the NaZn₁₃ crystal structurefor the entire cobalt content of 3.9 weight percent.

These sets of embodiments illustrate that the sinter activity alsodepends on the carbon content as well as on the silicon content.Consequently, the silicon content should be adjusted for a given carboncontent required to provide a given T_(c) as well as to provide auniform sinter activity and uniform resulting density of the workingcomponent. This hinders the cracking and/or delamination of portions ofthe working component. A similar result is expected by adjusting thealuminium content in dependence of the carbon content.

In a further group of embodiments, the effect of varying the silicon andaluminium contention the sinter density was investigated.

Table 14 summarizes the compositions of six samples of varying Si and Alcontent. The total content of silicon and aluminium remains largelysimilar, but the proportion of silicon to aluminium is varied so thatsample 1 includes only aluminium and no silicon and sample 6 includesonly silicon and no aluminium.

Table 15 summarizes the density of the samples of Table 14 after heattreatment at different temperatures in the range of 960° C. to 1110° C.These results are also illustrated in the draft of FIG. 17.

These results indicate that the aluminium content can also be adjustedto adjust the density of the working component. As the silicon contentincreases and the aluminium content decreases, the sinter temperatureshould be increased to achieve a high sinter density.

In a further group of embodiments, the effect of aluminium on the sinterdensity for samples with no silicon, i.e. silicon-free samples, wasinvestigated.

Table 16 summarizes the compositions of six samples of varying Alcontent. The samples were heat treated at temperatures between 940° C.and 1040° C. The sinter density of these samples is summarized in Table17.

FIG. 18 illustrates a graph of sinter density in dependence of sintertemperature for samples with varying aluminium contents.

The results indicate that for increasing Al content, the sintertemperature has to be increased in order to achieve a high sinterdensity. This trend is comparable to the embodiments in which thesamples only contain silicon and no aluminium.

The invention claimed is:
 1. An article for magnetic heat exchange,comprising: a monolithic sintered working component comprisingLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) with a NaZn₁₃ structure,wherein M is one or more element selected from the group consisting ofSi and Al, T is one or more element selected from the group consistingof Mn, Co, Ni, Ti, V and Cr, R is one or more element selected from thegroup consisting of Ce, Nd, Y, and Pr, 0≤a≤0.5, 0≤b≤1.5, 0.05≤x≤0.2,0≤y≤0.2, and 0≤z≤3, a content of at least one of the elements selectedfrom the group consisting of T, R, C, and M varying in a workingdirection of the working component, a Curie temperature monotonicallydecreasing or monotonically increasing in the working direction of theworking component, and the working component comprising an averagegradient of the Curie temperature of 5° C./mm to 0.5° C./mm over 80% ofa length of the working component.
 2. The article according to claim 1,wherein the Curie temperature increases or decreases with a gradientthat lies within ±50% of a linear function over 80% of a length of theworking component, and the linear function is defined as the differencebetween the Curie temperature in degrees Celsius at one end of theworking component and the Curie temperature at an opposing end of theworking component divided by a distance in millimeters between the twoends.
 3. The article according to claim 2, wherein the Curie temperaturedecreases or increases with a gradient that lies within ±5% of thelinear function.
 4. The article according to claim 2, wherein the Curietemperature decreases or increases with a gradient that lies within ±10%of the linear function over 90% of the length of the working component.5. The article according to claim 1, wherein the working component has adensity d in a defined portion of 5 vol % to 10 vol % of a total volumeof the working component and wherein the density d lies within a rangeof ±5% of an average total density d_(av) of the working component. 6.The article according to claim 1, wherein the working componentcomprises an increasing M content in the working direction of theworking component or a decreasing M content in the working direction ofthe working component for increasing amounts of one or more of theelements T and R in the working direction of the working component. 7.The article according to claim 1, wherein T of the working componentcomprises Co and/or Mn, M comprises Si, and a Si content, Si_(act),lying within ±5% of Si_(m), whereinSi_(m)=3.85−0.0573×Co_(m)−0.045×Mn_(m) ²+0.2965×Mn_(m), wherein Si_(m)is a metallic weight fraction of Si, Mn_(m) is a metallic weightfraction of Mn, and Co_(m) is a metallic weight fraction of Co.
 8. Thearticle according to claim 1, wherein nowhere along a length of theworking component is there a gradient of the Curie temperature thatexceeds 10° C./0.5 mm of the length.
 9. An article for magnetic heatexchange, comprising: a monolithic sintered working component comprisingLa_(1-a)R_(a)(Fe_(1-x-y)T_(y)M_(x))₁₃H_(z)C_(b) with a NaZn₁₃ structure,wherein M is one or more element selected from the group consisting ofSi and Al, T is one or more element selected from the group consistingof Mn, Co, Ni, Ti, V and Cr, R is one or more element selected from thegroup consisting of Ce, Nd, Y, and Pr, 0≤a≤0.5, 0≤b≤1.5, 0.05≤x≤0.2,0≤y≤0.2, and 0≤z≤3, a content of at least one of the elements selectedfrom the group consisting of T, R, C, and M varying in a workingdirection of the working component, and a Curie temperaturemonotonically decreasing or monotonically increasing in the workingdirection of the working component, wherein nowhere along a length ofthe working component is there a gradient of the Curie temperature thatexceeds 10° C./0.5 mm of the length.
 10. The article according to claim9, wherein the Curie temperature increases or decreases with a gradientthat lies within ±50% of a linear function over 80% of a length of theworking component, and the linear function is defined as the differencebetween the Curie temperature in degrees Celsius at one end of theworking component and the Curie temperature at an opposing end of theworking component divided by a distance in millimeters between the twoends.
 11. The article according to claim 10, wherein the Curietemperature decreases or increases with a gradient that lies within ±5%of the linear function.
 12. The article according to claim 10, whereinthe Curie temperature decreases or increases with a gradient that lieswithin ±10% of the linear function over 90% of the length of the workingcomponent.
 13. The article according to claim 9, wherein the workingcomponent has a density d in a defined portion of 5 vol % to 10 vol % ofa total volume of the working component and wherein the density d lieswithin a range of ±5% of an average total density day of the workingcomponent.
 14. The article according to claim 9, wherein the workingcomponent comprises an increasing M content in the working direction ofthe working component or a decreasing M content in the working directionof the working component for increasing amounts of one or more of theelements T and R in the working direction of the working component. 15.The article according to claim 9, wherein T of the working componentcomprises Co and/or Mn, M comprises Si, and a Si content, Si_(act),lying within ±5% of Si_(m), whereinSi_(m)=3.85−0.0573×Co_(m)−0.045×Mn_(m) ²+0.2965×Mn_(m), wherein Si_(m)is a metallic weight fraction of Si, Mn_(m) is a metallic weightfraction of Mn, and Co_(m) is a metallic weight fraction of Co.
 16. Thearticle according to claim 15, wherein Si_(act) lies within ±2% ofSi_(m).
 17. The article according to claim 9, wherein 0.05≤b≤0.5. 18.The article according to claim 9, wherein 1.4<z≤3.
 19. The articleaccording to claim 9, wherein T is selected from one or more of thegroup consisting of Co and Ni and wherein z=0.
 20. The article accordingto claim 9, wherein T is selected from one or more of a group consistingof Mn, Ti, V and Cr, R is selected from one or more of a groupconsisting of Ce, Nd and Pr, and wherein 1.4<z≤3.